A theory of field emission was first discovered by R. H. Fowler and L. W. Nordheim in 1928; by applying a strong electric field between a field emission cathode and a field emission anode, the electrons might be able to directly tunnel through a potential barrier from the field emission cathode and to collided to the field emission anode. Based on this theory, in practice, the field emission cathode can be constituted by using outgrowing spike structures in the past, or carbon nano tubes (CNT) or nano zinc oxide materials nowadays. These structures or materials being used nowadays have better aspect ratio, thereby generating greater field emission enhancing factors for enhancing field emission.
Carbon nano tubes are single or multiple layered nano scaled graphite sheets forming a hollow cylindrical structure. Because of the small diameter and the large depth-to-width ratio of the carbon nano tube, hundreds to thousands times of locally enhanced electric field can be generated at the tip of the carbon nano tube, so that electrons can be emitted with an electric field of 1˜2 V/μm from the CNT by overcoming a work function of 4.5 eV, which offers an excellent electron-emitting effect and can be applied to field emission lighting components in the field of light emitting. When the carbon nano tube is disposed at the cathode of an electric field, electrons can be driven to emit from the tip of CNT by the driving force of the electric field. Those electrons will be collided with phosphors layer on the anode through a vacuum interval, thereby a light beam is emitted from the phosphor layer based on the field emission light theory. The field emission light theory is applied for developing field emission light (FEL), and field emission display (FED). For instance, in 2002, J.-M. Bollard, R. Gaal, S. Garaj et al. had published a paper (Field emission properties of carbon nanohorn films; Journal of applied physics 91 (12): 10107-10109), which illustrates that carbon nanostructure, multi-single-wall carbon nano tube, and carbon nano cone have great field emission properties. In addition, US Pub. No. 20030001477, U.S. Pat. No. 7,276,843, and a paper published by Pan L, Hayashida T, Nakayama Y et al. in 2002 (Fabrication of Carbon Nanocoil Field-Emitters and Their Application to Display; Japan Hardcopy Vol 2002, page 533-534, 2002) also illustrate that single-wall carbon nano tube, cylindrical graphene, graphitic nano fibers, carbon nano-coil fiber can be used as a cathode emitter.
Field emission is driven by electrical energy to emit light; nowadays, field emission lighting sources are gradually sprout in the field of lighting. Referring to FIG. 1, a field emission cathode 95 is disposed in a vacuum glass tube 92 of a field emission lighting source 91, a electric field is formed while applying a electrical potential to the field emission light source 91, so that a light can be generated by colliding an electron beam emitted from the field emission cathode 95 with a phosphor layer 941, where the luminous efficiency thereof ranges from 40 to 60 lumens per watt. By using this kind of field emission light source having simple structure and having none energy consuming during semiconductor manufacture, the luminance environment in human society may be significantly improved.
The major component in the field emission lighting source is the field emission cathode; in numerous prior arts, processes for fabricating carbon nano tubes being used in the field emission cathode have been provided, such as the chemical vapor deposition (CVD) method provided by J-Ean-Bonard and Thomas stockli et al. in 2001 (Appl. Phys. Lett. 78, 2775), in which a metal filament made of Kanthal (Fe—Al—Cr alloy) was used as a substrate and was immersed in a catalyst of Fe(No3)39H2O, and then the substrate was disposed in a reacting furnace with a temperature of 720° C. and acetylene and nitrogen gas were introduced thereinto. By using the CVD method, carbon nano tubes were then being grown on the surface of the metal filament, and forms a field emission cathode filament; while this field emission cathode filament is combined to an anode fluorescence tube, a field emission lighting source can be formed. This process for fabricating CNT directly by using CVD, however, needs to be processed in a high temperature environment, and those instruments used in this process are expensive. Additionally, using this process to mass produce the field emission lighting source is quite difficult and is not cost-effective. Moreover, the carbon nano tube is linear-shaped, electrons can only be emitted from the tip end thereof, and the emitting area is then limited accompanying with the melting of the carbon nano tube, so that the luminous quantity is low, thereby limiting the range of its applications.
Other processes each including a step of depositing a conductive layer on the substrate for growing CNT were also provided and disclosed under patents or publications, such as U.S. Pat. No. 6,948,995, TW Pat. No. I321806, or a paper published by Woo Yong Sung et al. in 2007 (Nanotechnology 18 245603) where CNT was grown on an electrical conductive thin layer on a nickel metal substrate. In addition, in 2008, Weiqi Fu, Liang Liu et al. had also provided a process (J. Vac. Sci. Technol. B, vol. 26, 1404), in which a stainless steel sphere was applied to be as a substrate, and this substrate was first coated with a silver paste. Then, a slurry containing carbon nano tube combined with organics which is like ethyl cellulose, terpineol, and diethyl ophthalate, etc. were coated on the surface of the substrate followed by a baking step. Next, the substrate was fixed on an inner center of an anode fluorescence sphere in order to form a field emission lighting bulb. Moreover, in TW Pat. No. I318963, another process was disclosed, in which a carbon nano tube array and prepolymer of polymethyl methacrylate thin layer was disposed on a substrate for fabricating materials of field emission cathode. In JP2005-166690, U.S. Pat. No. 7,960,904, an electrical conductive paste was plated on a field emission cathode lamp filament for adhering with carbon nano tube. Nevertheless, while using these processes, the requirement of an amount of organic materials like diethyl opthalate, polymethyl methacrylate is great such that organics will remain. Therefore, while fabricating the field emission lighting source, the organics will gradually leak during vacuum packaging and ion bombard, thus influencing the working life and the service luminous efficiency of the field emission lighting source.
In still another process disclosed under prior arts is to form composited plated layer by whether electroplating or chemical plating, such as in U.S. Pat. No. 6,975,063, Mao et al. disclosed an electroless plating manner, in which a carbon nano tube and a reduced metal were co-deposited on the surface of a substrate, and then the composited plated layer of carbon nano tube and metal can be obtained, and therefore the adhesion between the substrate and the carbon nano tube can be improved. However, in this manner, a traditional composited plating process was used, the plating solution thereof is easy to decompose and is hard to be storage. Plus, the emission efficiency and the luminance uniformity of the field emission lighting source merely using the carbon nano tube for field emission was too low to be used widely.
Another process as TW Pub. No. 201025415 and TW Pub. No. 201001476 were disclosed; in which zinc, zinc-plated, or aluminum substrate was roughened, afterwards, the substrate surface was immersed in a solution containing single-wall carbon nano tube, double-wall carbon nano tube, wall-less carbon nano tube, multi-wall carbon nano tube, carbon nano fiber, coil shaped carbon nano fiber, nano diamond, or other carbon nano materials, so that the surface of the substrate can be coated with a layer of nano carbon materials in order to fabricate a field emission cathode. Among these prior arts, even the carbon nano fiber had better field emission characteristics than the carbon nano tube, the fractional yield of the carbon nano fiber was low, and the aspect ratio of the carbon nano fiber was small due to the rapid growth of the carbon nano fiber triggered by metals like nickel, zinc, or aluminum, so that when the carbon nano fiber is applied to be as a field emission cathode, the overall electron beams emitted from the field emission cathode are still insufficient.
Other prior arts such as U.S. Pat. No. 6,333,016 and US Pub. No. 20020090468 have provided processes for fabricating carbon nano tubes, in which reduced metals were used to germinate carbon nano tubes, and more particularly, catalytic particles contain at least one metal from Group VIII, and at least one metal from Group VIb as assisted metal catalysts for germinating the carbon nano tubes. Furthermore, manners of chemical replacement of metals were also disclosed under TW Pat. No. I274789 and TW Pub. No. 200743677 for fabricating carbon nano tubes, where composited metal micro-particles such as nickel-cobalt, nickel-gold, nickel-platinum, cobalt-palladium or cobalt-platinum were used for growing carbon nano tubes. However, reduction reactions of these manners using metal catalysts and assisted metal catalysts were difficult to control, and the uniformity of the carbon nano tubes grown by using these manners was insufficient, such that field emission generated by these manners was poor, thereby the luminance quantity of the field emission lighting source was unlikely to be enhanced.
What is more, TW Pub, No, 201014787 and a paper published by J. X. Hung, Jun Chen et al. (J. Vac. Sci. Technol. B, vol. 26, 1700) disclosed that by using the thermal chemical vapor deposition process, where stainless steel filaments was used as a substrate, and iron, cobalt, nickel, rhodium, palladium, or platinum was used as a catalyst, carbon nano tubes can be formed with high sintering temperature for fabricating field emission cathode filaments. These field emission cathode filaments can be combined with an anode fluorescence tube to form a field emission lighting tube. According to some disclosures, including U.S. Pat. No. 7,098,112, US Pub. No. 20050023950, and JP2008-254757, nano carbon materials, majorly carbon nano tubes, can be grown on a silicon cathode carrier with cobalt catalysts. Besides, except the carbon nano tubes, other nano materials grown on the silicon carrier are also included such as carbon nano fibers, carbon nanohorns, carbon nanofilament walls, and so on. In US20060142148, US20110014368, U.S. Pat. No. 7,923,403, U.S. Pat. No. 7,396,798, and U.S. Pat. No. 7,968,489, palladium, oxidized metals, or sulfurized metals may be used as catalyst for growing carbon nano tubes, single-wall carbon nano tubes, or graphene. Moreover, TW Pub. No. 201012964 has disclosed that unsaturated vinyl monomers and hydrophilic monomer copolymer of noble metal catalysts were used to grow carbon nano tubes. However, the final yield of the coiled carbon nano tubes and coiled carbon nano fibers grown by using these processes are insufficient, and the diameters of the carbon nano tubes are about 200 nm˜300 nm as well as the aspect ratios of carbon nano tubes are too high to use in field emission. Accordingly, carbon nano tubes with relatively smaller aspect ratios were disclosed under U.S. Pat. No. 8,044,581, where the diameter and lengths of such carbon nano tubes ranges from 5˜10 nm and 20 nm˜1 μm, respectively. As above, carbon nano tubes are linear shaped, and electrons can only be emitted at the tip of the carbon nano tubes, so that the electron beams emitted therefrom are limited, even the intensity of the electron beams might be enhanced by increasing the emitting electric potential during field emission, the working life of the field emission lighting source would be significantly shortened as a side effect.
Hence, it is the major issues of field emission lighting sources to grow coiled-structured nano carbon materials applied to field emission cathodes with high fractional yields, and to be endure the consequence of applying higher driving electric potentials to provide higher luminance quantities.